This invention relates to internally coated, threaded end, tubular goods, and more particularly to tubular products such as pipe, tubing, casing and the like used in corrosive environments in the oil and gas drilling and production industries.
It has long been common practice where corrosive environments are encountered in the oil and gas industries to use internally coated tubular elements and collars, so as to avoid contact between the corrosive gases and liquids and the metal surfaces. The coating materials and processes are costly, but despite constant development and careful control minute defects in the plastic coating still cause excessive problems. For example, pinholes, fracture points and protruding metallic whiskers can be present that act as sites for attack by the corrosive materials. The most frequent and troublesome problems, however, arise at the nose ends of inserted pins. Internal coating machinery cannot thoroughly cover the nose end of the pipe without special adaptation. In any event any sharp corner, junction or discontinuity between two different surfaces presents a weak point in the coating system. In practice, coated tubular goods taken from a well after prolonged exposure are very often found to have been significantly corroded, even substantially eaten away, at the nose ends.
In the oil and gas industry any defect that can serve as a corrosion side in a coated tubular product is referred to as a "holiday". The search for "holiday free" processes and products has been continuous but has not resolved the pin end corrosion problem. For the interior body of a tubing or casing, electrical inspection equipment passed through the product can sensitively respond to pinholes, apertures and protruding metal points. While the coating may have to be removed so that a new surface can be applied when a defect has been detected, there is a degree of assurance available. The same is not true of the problems that arise at ends and corners, however. Here the physical discontinuities and the need for secure makeup when threading together may be the cause of a break in the coating, as may a thin or non-adhering area at the corner or end.
An important reason that seals, rings or other added elements in abutment with the nose ends have not been satisfactory derives from other requirements or conditions. For example, wall thickness, particularly in the threaded region of the pin, must be sufficient to withstand the tensile and circumferential stresses induced when using long strings in high pressure internal or external conditions. In the makeup of couplings, some cone distortion results from the high interference levels applied and this initial radial stress limits what can be resisted from other sources. Moreover, the makeup problem also affects the designs that can advantageously be used with coated pipe. Even if torque measuring equipment is used, with the attendant slowing of the makeup sequence at considerable expense, corrosion problems arise from inadequate or excessive bearing pressures. Not the least of these problems arises from the fact that excessive forces and pressures can introduce local deformations that coatings cannot follow without cracking.
A technique used in certain couplings for reduction of electrolytic corrosion is worthy of note here, as evidencing an attempted solution for a different problem. Where the confined gases and liquids have electrolytic properties they can fill the space between pipe ends and produce a galvanic cell that causes pipe end corrosion. Therefore, workers in the art have placed corrosion barrier rings between the opposed pipe ends, compressing the rings during makeup but interposing an insulative gap. In one form these rings have planar side faces, and in another they include lips that fit under the side taper adjacent the pipe nose. Neither configuration has been much used in practice, probably because they both tend to become compressed from the ends so as to protrude into the fluid stream, which pulls them out. They also interface with tools that must be lowered into and retrieved from the well bore. It is difficult to control coupling makeup so that the corrosion barriers, which are not sealing elements, are not unduly distorted. The seal elements can be permeated slowly by gases during operation under high pressure, creating limited volume traps against the tube wall. If the interior is suddenly depressurized, the resultant differential pressure forces the seals inwardly into the interior and at times even results in explosive decompression of the seal or barrier material.